The present invention relates to a multi-segment filter system for continuous filtration of high viscosity materials, such as polymer melts. Each segment includes a filter screen disposed adjacent a breaker plate and cover plate to secure the filter screen in a fixed alignment. The system allows for continuous filtration while minimizing the occurrence of unwanted gaps that retard or disrupt the filtration process.
One common method for the manufacture of plastic articles includes heating a polymeric material to an appropriate temperature to form a polymer melt which can then be injected into a mold and cooled such that the plastic article adopts the shape of the mold (i.e. injection-molding).
To obtain a high quality molded article, it is common to purify the polymer melt via filtering, prior to injection-molding. For example, if an unwanted particle is introduced into the injection mold, a pinhole may form in the article. Then if the molded article is subsequently blow-molded, the pinhole may cause the blow-molded article to fracture or explode. A purification/filtration step can be combined with melt extrusion, whereby an extruder forces the polymer melt through a filter under high pressure and the resulting filtered polymer is then introduced into the mold.
Filtration can pose several difficulties, often due to hardening of the polymer melt (i.e. increased viscosity upon cooling). Polymeric residues may stick to a filter and harden over time, resulting in a blocked filtration pathway. Filtered impurities can also block the filter and retard the filtration process. When a blockage occurs, higher pressure is required to extrude the polymeric material through the blocked filter. This higher pressure may disrupt the extrusion process and/or strain the machinery. In contrast, a predictable and constant pressure drop across the filter over time allows for a smooth and relatively uninterrupted extrusion.
The above difficulties are somewhat alleviated by the use of a continuous filtration system. A xe2x80x9ccontinuous filtration systemxe2x80x9d provides either a roll of filtering material i.e., a very long filter screen, or a plurality of individual filter screens (segments) positioned in a line (i.e., adjacent one another on end). The filter screen segments or roll move transversely across the exit end of the extruder, in a direction perpendicular to the extruded melt flow. The movement of the screens or roll can occur either at a constant steady rate, or intermittently. In this manner, polymer melt is constantly exposed to a clean portion of a filter screen roll, or a new filter screen. Accordingly, this system decreases the likelihood of exposing the melt to a blocked filter. As a result, consistent pressures (constant pressure drop) can be applied to force melt through the filter, providing for ease of operation and reducing the number of delays and amount of shutdown time.
Problems with known multi-segment continuous filtration systems remain however, particularly in preventing the formation of gaps between the filters that allow small amounts of polymer melt to be injection-molded as unfiltered material. For example, the high pressures applied to polymer melts during filtration may cause a filter screen segment to shift from its desired position. Intermittent extrusions, which involve abrupt stops and starts, can cause additional problems. For example, in an intermittent extrusion process a high pressure polymer melt stream is followed by a complete removal of pressure when the extruder is pulled back (in a direction opposite to the filter). As the extruder is pulled away, the polymeric material contacting the filter screen may cause the screen segment to lift and shift its position, resulting in the formation of a large, unwanted gap. In a subsequent extrusion step, unfiltered material may then pass through this gap.
Other prior art systems utilize a rotatable filtration device, namely a plurality of screens positioned in a circular pattern. Such devices are not easily cleaned, because the cleaning step involves back-filtering pure polymer melt through the filter to release impurities.
Thus, there remains a need for improved continuous filtration devices.
One aspect of the present invention is a system for continuous filtration comprising multiple segments. Each segment comprises a filter screen, a breaker plate positioned adjacent one face of the filter screen, and a cover plate positioned adjacent an opposing face of the filter screen. The screen is held in fixed alignment with respect to the breaker and cover plates.
In another aspect of the invention, a leading breaker plate has a portion of a first filter screen disposed thereon, and an end portion which provides an overhang. A trailing breaker plate has a second filter screen disposed thereon. The leading and trailing breaker plates are positioned adjacent on end, such that the overhang of the first screen overlaps with an end portion of the second screen.